The present invention has for its object to provide a mixed device for generating longitudinal and transverse or shear waves, particularly adapted to land seismic prospecting.
In seismic prospecting, the location of the reflecting layers of the subsoil is determined by transmitting through the earth mainly longitudinal acoustic waves (P waves) which propagate according to the compression mode. In addition, there is also used transverse or shear waves (S waves) which propagate according to a distortion or shear mode. The shear waves have interesting applications since their wave length is, at equal frequency, lower than that of the longitudinal waves and they have a better resolving power. In addition, some of them (SH waves) have the particularity, in certain conditions, of being not subjected to any conversion in the presence of stratigraphic discontinuities and the recordings obtained from said waves are simpler.
The seismic prospecting methods comprising the combined transmission of longitudinal and shear waves permit obtaining a more precise knowledge of the sub-soil but two separate seismic sources are necessary for their use, particularly sources specialized for generating one of these two waves types.
A known device adapted to generate acoustic shear waves is described for example, in the French patent No. 2,276,599 corresponding to U.S. Pat. No. 4,118,994. It essentially comprises a target member carried on a vehicle, which can be coupled to the ground intermittently, a weight for striking the target member and orientable movable driving means for laterally lifting the weight and orienting it fall towards the vertical lateral walls of the target member when the latter is in contact with the earth.
A known device adapted to generate acoustic longitudinal waves is described, for example, in the French patent No. 2,398,316 corresponding to U.S. Pat. No. 4,205,731. It essentially comprises a target secured at a first end of a guide tube and adapted to come in contact with the earth when the tube is substantially vertical, a weight freely slidable in the tube and adapted to impinge on the target, means for moving said weight up to an upper position at the second end of the tube and means for retaining it intermittently in said position. The means for moving the mass up to its upper position after each fall comprises, for example, a system integral with a rigid support for pivoting the tube between the vertical position of operation and a second position where the first end of the tube is above the opposite end.
A mixed device for successively generating in the earth, at the same location, longitudinal or shear waves in order to avoid the use of two separate seismic sources has been proposed in the U.S. Pat. No. 4,244,437. This device comprises a hoisting mast for displacing a weight with respect to a target member adapted, in vertical position, to generate longitudinal waves by striking vertically the target member and which may be inclined to the vertical so that the weight strikes a lateral face of the target member to generate shear waves.
This system however suffers from serious drawbacks. As a matter of fact, the force exerted in the direction of the target is reduced with respect to the gravity force of the weight, on the one hand by a factor corresponding to cos .alpha., .alpha. being the inclination angle of the hoisting mast to the vertical, and, on the other hand by the friction forces developed by the motion of the weight with respect to its guiding means on the hoisting mast.
Moreover, this force, exerted in the direction of the target is used only to a partial extent since only its horizontal component may generate shear waves, this horizontal component being derived from the force available in the direction of the target, by application of a coefficient equal to sin .alpha.. Finally, the useful force for generating shear waves is at most equal to one half of the gravity force of the weight since cos .alpha..multidot.sin .alpha.=1/2 sin 2.alpha.